All optical communication systems include three major building blocks: a source, an optical fiber, and a detector. The source is usually a semiconductor laser.
A semiconductor laser needs both gain and feedback to operate. A typical laser is manufactured in a semiconductor wafer. When the laser is broken away or "cleaved" from the wafer, cleaved facets are formed on the front and rear of the laser. The cleaved facets provide some or all of the required feedback for the laser. The feedback from the cleaved facets may in some instances be tailored by, for example, adding optical coating.
There are multiple types of semiconductor lasers. One type of laser is referred to as a "Fabry-Perot" laser. A Fabry-Perot laser is a multi-mode laser that receives all of its feedback from its cleaved facets. Another type of laser is referred to as a "Distributed Feedback" ("DFB") laser. FIG. 1 illustrates an example of a typical DFB laser 10. DFB laser 10 includes an active region 16 and front and rear facets 12 and 14. Facet 12 provides a reflection 13 and facet 14 provides a reflection 15. DFB laser 10 further includes a diffractive grating 18. Diffractive grating 18 provides additional reflection 19 and 20. Thus, DFB laser 10 receives feedback from both its facets and diffractive grating. A DFB laser is a single-mode laser.
The spectral properties of a DFB laser are very dependent on the strength of the interaction of the lasing light with the diffractive grating. The strength of the interaction is referred to as KL.
When manufacturing DFB lasers or other electro-optic devices, multiple devices are typically fabricated on a single wafer. Determining the yield of the wafers generally requires a considerable amount of individual device testing to select or screen "good" performing devices from the devices with inadequate performance or, in some cases, higher grade devices from lower grade ones. Currently most of this testing or screening is done on devices after they have been separated from the wafer and, for a DFB laser, considerable amount of device to device variation is introduced by the cleaved facet itself. The individual device testing, often required on 100% of the devices, adds considerable expense to the cost of high performance laser devices.
Reduction in testing time and cost can be achieved by selecting for subsequent processing only the wafers which are likely to provide the higher yields at testing. Time and inventory could be saved in this wafer certification process if the laser devices could be evaluated or sampled while in wafer form, i.e., prior to separation. Unfortunately, wafer level testing has not been well exploited in laser manufacturing because the light that is so important to the electro-optic properties of the laser quality is generally inaccessible until after the devices have been cleaved and separated from the wafer.
Based on the foregoing, there is a need for a method and system that provides testing of lasers before they are separated from the wafer.